Pacific Islands Water Science Center
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The order of accounting for evapotranspiration and recharge in threshold-type water budgets can have an effect on ground-water recharge estimates, which is a concern for evaluating water availability. For monthly water budgets, accounting for recharge before evapotranspiration is most appropriate in areas where rainfall occurs infrequently (such as southwest O‘ahu, top), whereas accounting for evapotranspiration before recharge is most appropriate where rainfall occurs relatively uniformly throughout the month (such as the eastern part of the Island of Hawai‘i, bottom). USGS photos by D.S. Oki (top) and D.C. Nishimoto (bottom).
Most threshold-type water-budget models account for the loss of water by evapotranspiration before accounting for recharge. Recharge estimates can differ substantially, depending on whether recharge is counted before or after evapotranspiration in the water budget. This disparity is the source of uncertainty and is most pronounced for areas where soil-moisture storage capacity is small or for water budgets computed using a large time interval (such as monthly). Water budgets that account for recharge before evapotranspiration provide higher estimates of recharge and lower estimates of evapotranspiration relative to water budgets that account for evapotranspiration before recharge. The choice of accounting method is less significant for a daily computation interval than for a monthly computation interval. In general, uncertainty in recharge estimates is least for water budgets computed using the shortest computation interval that the data allow and that is consistent with the physical processes being represented. If the data only allow for long (weekly or monthly) computation intervals, then selecting the appropriate accounting order for the study area may be critical. For monthly water budgets, accounting for recharge before evapotranspiration is most appropriate in areas where rainfall occurs infrequently, whereas accounting for evapotranspiration before recharge is most appropriate where rainfall occurs relatively uniformly throughout the month.
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